There has been proposed an image forming apparatus that has a function of correcting misalignment of image forming positions on a sheet, for example. More specifically, an image, such as a registration pattern having a plurality of marks, is formed on a surface of a belt. A detection sensor of an optical type, whose detecting region is defined on the surface of the belt, is employed. Positions of the marks are detected based on a light reception signal outputted from the detection sensor. Misalignment of image forming positions are corrected based on the detection results.
However, a measurement error possibly occurs between the positions of the marks detected by the detection sensor and the actual positions of the marks. This measurement error degrades the accuracy in correcting the image forming positions. Japanese Unexamined Patent Application Publication No. 2007-025315 has noted that there is certain relationship between the amount of the measurement error and the amount of distortion that is generated in the detection waveform due to a diffuse reflection light. The publication describes that the measurement error is determined by measuring the amount of distortion in the detection waveform.